This invention relates to a novel integrated process for removing hydrocarbon and other organic contamination from feed gas streams for Claus reactors. This invention also relates to the use of unique inorganic molecular sieves of the type containing octahedrally coordinated metal sites, such as coordinated octahedrally titanium, in processes for removing hydrocarbon and other organic contamination from hydrogen sulfide-containing streams.
Natural gas as well as refinery gas streams are commonly contaminated with sulfur compounds, especially hydrogen sulfide (H2S). If substantial amounts of hydrogen sulfide are present, regulatory restrictions dictate special precautions must be taken to purify the gas streams. In non-polluted areas, generally a maximum of two tons per day of sulfur are allowed to be vented as sulfur oxide (SO2) flare-off gas per processing plant. In populated areas even more stringent restrictions are applied.
The first step in H2S removal from natural gas and/or refinery streams is accomplished by an acid gas removal unit. This unit removes substantial amounts of H2S and CO2 from the processing stream. The off-gas of this stream contains predominantly CO2 and H2S. The sulfur from this off-gas stream is removed by the Claus reaction which produces salable elemental sulfur. The remaining CO2 may be safely vented to the atmosphere.
The Claus process was discovered over 115 years ago and has been employed by the natural gas and refinery industries to recover elemental sulfur from hydrogen sulfide-containing gas streams for the past 50 years. Briefly, the Claus process for producing elemental sulfur comprises two major sections. The first section is a thermal section where H2S is converted to elemental sulfur at approximately 1,800-2,200xc2x0 F. No catalyst is present in the thermal section. The second section is a catalytic section where elemental sulfur is produced at temperatures between 400-650xc2x0 F. over an alumina catalyst. The reaction to produce elemental sulfur is an equilibrium reaction, hence, there are several stages in the Claus process where separations are made in an effort to enhance the overall conversion of H2S to elemental sulfur. Each stage involves heating, reacting, cooling and separation. A flow diagram of the Claus process is shown in FIG. 1 which will be explained in more detail below.
In the thermal section of the conventional Claus plant, a stoichiometric amount of air is added to the furnace to oxidize approximately one-third of the H2S to SO2 and also burn all the hydrocarbons and any ammonia (NH3) present in the feed stream. The primary oxidation reaction is shown as follows:
2H2S+3O2xe2x86x922SO2+2H2Oxe2x80x83xe2x80x83(1)
This reaction is highly exothermic and not limited by equilibrium. In the reaction furnace, the unconverted H2S reacts with the SO2 to form elemental sulfur. This reaction is shown as follows:
xe2x80x832H2S+SO2⇄3S0+2H2Oxe2x80x83xe2x80x83(2)
Reaction (2) is endothermic and is limited by equilibrium.
In the catalytic section of the Claus process, the unconverted hydrogen sulfide and sulfur dioxide from the thermal stage are converted to sulfur by the Claus reaction (2) over an alumina catalyst. Typically, there are three stages of catalytic conversions. Important features of the Claus reaction in the catalytic stage are that the reaction is equilibrium limited and that the equilibrium to elemental sulfur is favored at lower temperatures.
The Claus process was modified in 1938 by I.G. Fabenindustrie and various schemes of the modified process are utilized today. For feed gas streams containing approximately 40% H2S, the balance carbon dioxide (CO2) and water (H2O), the once through Claus process is generally employed in which all of the acid gas is fed directly to the burner. Three catalytic stages are typically utilized after the initial thermal stage. This scheme will generally produce an overall recovery of 95-97% sulfur. If this recovery efficiency is acceptable, no further processing is required. However, if the recovery efficiency is not high enough (for a variety of reasons and, in particular, environmental constraints) an advanced Claus process such as Comprimo""s Super Claus process which has a sulfur efficiency of 99.0% can be utilized. This process consists of the replacement of the final Claus reaction stage by, or the addition of, a reaction stage featuring a proprietary catalyst to promote the direct oxidation of hydrogen sulfide to sulfur selectively in the Claus tail-gas. Air is injected upstream of the reactor. The hydrogen sulfide and oxygen react over the catalyst via the following reaction:
2H2S+O2xe2x86x922S0+2H2Oxe2x80x83xe2x80x83(3)
If a sulfur recovery efficiency of greater than 99% is required, a tail-gas cleanup unit (TGCU) needs to be employed. This type of unit allows for an overall sulfur recovery efficiency of 99.8%. In the United States, a sulfur recovery efficiency of 99.8+% is required for Claus production units generating greater than or equal to 50 STSD of elemental sulfur, hence, a TGCU such as the Shell Scot process is often required. Such processes coupled with a sulfur recovery unit (SRU) can meet and exceed a sulfur recovery efficiency of 99.8+%.
There are other modifications to the basic Claus process. One particular modification to the Claus process that is widely used today is the xe2x80x9cSplit-Flowxe2x80x9d process for feed gas streams containing 30-35% H2S or less concentrations. In this scheme, 40-60% of the feed gas is passed directly to the catalytic section, bypassing the noncatalytic reaction furnace. This process is utilized to achieve a hotter temperature and a more stable flame in the furnace. The bypassed feed joins the furnace effluent after the condenser and the combined flow enters the first catalytic converter. The sulfur recovery efficiency for this scheme is normally 1-3% lower than the conventional once-through or straight-through process. Basic descriptions of Claus process schemes and additional tail-gas cleanup units are given in the Kirk Othmer Encyclopedia of Chemical Technology, Vol. 23, pp. 440-446, the contents of which are incorporated herein by reference.
In the Claus reaction scheme, it can be seen that combustion air is a critical variable in maintaining a high efficiency operation in the thermal section. Hydrocarbon impurities and other feed gas contaminants not only cause a high temperature operation (up to 2,500xc2x0 F.) such contaminants cause problems in maintaining the correct amount of combustion air. Additionally, it should be noted that in the first catalytic stage, any carbonyl sulfide (COS) and carbon disulfide (CS2) that are formed in the reaction furnace and/or any such materials entering the catalytic section with the feed gas such as in the split flow process must be hydrolyzed to hydrogen sulfide and CO2 if they are to be removed. Any sulfur-in the form of COS or CS2 leaving the first catalytic stage cannot be recovered by the Claus process because of the lower temperatures used in the second and subsequent catalytic stages. A bottom bed temperature of 600-640xc2x0 F. is required in the first catalytic stage for good hydrolysis which in turn requires an inlet bed temperature greater than 500xc2x0 C. Normal operation for the inlet bed temperature is generally 450-460xc2x0 F., hence the higher temperature for the former does not favor the equilibrium to elemental sulfur formation.
In the Claus process design and operation to date, it is the design and operation of the reaction furnace, reaction furnace burner and the first catalytic converter or stage which are critical in an effort to achieve a successful operation. The burner is a critical piece of equipment in that it must be able to burn one-third of the incoming H2S while also burning all the impurities in the feed gas stream, namely, paraffin and aromatic hydrocarbons, ammonia and low molecular weight organics at substoichiometric air conditions. This is critical not only to the Claus unit where oxygen (O2) is detrimental to the alumina catalysts but also to the tail-gas cleanup units where a reducing condition is employed at the front end of the unit. In the design of the reaction furnace burner, there has been considerable discussion as to the type of burner to be utilized based solely on economics. More complex and expensive burners can handle moderately higher concentrations of hydrocarbon impurities and even higher molecular weight hydrocarbons, up to 1% propane. However, burner design, no matter how expensive, only addresses coping with the impurity and not solving the problem. In fact, the burner does not combust the lighter hydrocarbons, but the combustion products are mostly CS2 and CO2 and these compounds create additional problems that must be addressed. Also, when hydrocarbons are combusted, additional air is fed and CO2 and H2O are generated which adds to the volumetric flow which in turn requires larger equipment for a given sulfur production rate. Another problem is the fact that even the most expensive burner design cannot handle C4+ aliphatic hydrocarbons and all aromatic hydrocarbons. These materials can generate soot or polymeric hydrocarbons which can coat the reaction furnace and the first catalytic converter catalyst.
There are other problems associated with the presence of hydrocarbons in the Claus feed stream and consequent generation of CS2. The reaction of the hydrocarbons with H2S and O2 are endothermic in a furnace where an exothermic condition is required to generate a sufficient high temperature for their destruction. Additionally, in the first catalytic converter, any CS2 that is not hydrolyzed goes through the remaining part of the Claus unit as CS2 and presents a loss in sulfur recovery efficiency and a potential explosive hazard. As a case in point, the addition of 2% light hydrocarbon as methane (CH4) and ethane (C2H6) and 1.5% C6+ in the Claus feed results in a capital increase for the Claus plant of approximately 33%. Additionally, and also very important, the emissions as SO2 and CO2 increase by 25%.
It can be seen that hydrocarbon and other organic contamination of feed gas streams for Claus reactors, common in natural gas purification as well as in oil refinery processing, cause substantial processing problems. In addition to deactivating the Claus catalyst, organic species, when combined with sulfur, form a wide range of undesirable compounds. Many of these compounds are toxic and subject to strict regulatory restrictions. These regulations are driving efforts to identify appropriate means to remove the hydrocarbon and other organic contaminants before they reach the Claus reactor.
Adsorptive solutions to this hydrocarbon and organic contamination problem currently center on the use of activated carbons. However, the inability of activated carbons to completely reversibly regenerate results in excessive adsorbent consumption. After only a few cycles, the carbon must be disposed of and replaced because it rapidly loses adsorption capacity with each regeneration.
It would be very advantageous if an adsorbent could be identified which removed organic and other hydrocarbon contaminants from the highly polar acid gas stream which constitutes the Claus reactor feed. It would be especially advantageous if this adsorbent could be regenerated and reused through many cycles without substantial loss of adsorption capacity.
Adsorbents may be broken into two broad groups; those with a large quantity of specific, highly charged sites and those with large non-specific uncharged surfaces. Zeolites would represent a prime example of a xe2x80x9cspecificxe2x80x9d adsorbent and carbon and silica would represent prime examples of the xe2x80x9cnonspecificxe2x80x9d types. Specific site adsorbents may bind species very strongly, allowing for the essentially complete removal of favored trace components from larger streams. The sites in such materials bind with polar or polarizable species by electrostatic interaction. The bulk of Claus gas feed streams consist of highly polar H2O and H2S and extremely polarizable CO2. However, the sites in the specific adsorbent materials may be overwhelmed by the polar and polarizable species in such a stream and essentially a reduced number of sites would be available for binding with organics and hydrocarbons. Non-specific adsorbents tend to bind physically larger molecules on their surfaces and thus would be expected to selectively adsorb larger hydrocarbons from the combination of small molecules (H2O, H2S, CO2) which form the bulk of Claus feed streams. However, the weak binding energy inherent to non-specific adsorbents such as carbon substantially limits the adsorption capacity, especially of small hydrocarbons such as propane. Moreover, as discussed above, the non-specific adsorbents do not readily regenerate to the full original adsorbent capacity, and must be replaced after only a few adsorption/regeneration cycles.
It would be desirable to remove a broad spectrum of hydrocarbons in a Claus feed gas pretreatment system. An appropriate adsorbent would be a material which behaves like a non-specific adsorbent in the sense of favoring larger species such as organic and hydrocarbons while binding these with the high interaction forces and high selectivities associated with specific cited materials.
Importantly, it has been found that regardless of the adsorbent used, there is a level of H2S which is coadsorbed with the hydrocarbons. Without further processing, the adsorbed H2S would be present in the desorbed stream upon regeneration of the adsorbent This desorbed stream cannot be vented to the atmosphere or vented to a fuel system because of the residual H2S content. Further, coadsorption of H2S diminishes adsorbent activity for hydrocarbon removal resulting in the need for additional adsorbent requirement and consumption. Accordingly, the coadsorption of H2S represents an inherent problem in practicing the removal of hydrocarbons from a Claus feed stream using adsorption processing.
It has now been found that the unique three-dimensional framework of xe2x80x9cEXSxe2x80x9d molecular sieves, are particularly effective for the removal of organic compounds including hydrocarbons from hydrogen sulfide-containing feed gas streams for Claus reactors. EXS molecular sieves are distinguished from other molecular sieves by possessing octahedrally coordinated active sites in the crystalline structure. These molecular sieves contain electrostatically charged units that are radically different from charged units in conventional tetrahedrally coordinated molecular sieves such as in the classic zeolites. Members of the EXS family of sieves include, by way of example, ETS-4 (U.S. Pat. No. 4,938,939), ETS-10 (U.S. Pat. No. 4,853,202) and ETAS-10 (U.S. Pat. No. 5,244,650), all of which are titanium silicates or titanium aluminum silicates. The disclosures of each of the listed patents are incorporated herein by reference. The EXS sieves exhibit isotherms at temperatures slightly above ambient indicating the more active binding of organic species whereas at these temperatures, polar species show only minimal adsorption. As a consequence, organic species such as aliphatic and aromatic hydrocarbons can be selectively adsorbed from polar streams such as the feed gas stream to Claus reactors which contain polar species of H2S, CO2 and water.
Unlike the use of activated carbons, the organic species which have been adsorbed by the molecular sieves used in this invention can be removed by thermal or pressure swing processes reversibly for many cycles without significant loss of adsorption capacity. Accordingly, the present invention is further directed to a specific process of using, regenerating and reusing EXS molecular sieves for adsorbing organic species from hydrogen sulfide-containing or other polar gas streams.
The invention is also directed to a novel integrated process for removing hydrocarbons from a Claus feed stream using adsorption processing. In general, this invention effectively solves the problem of H2S coadsorption and the consequent process inefficiencies and environmental problems which result. The inherent problem of H2S coadsorption is solved in this invention by contacting the desorbed stream obtained from regeneration of the adsorbent with a lean acid gas removal solution either as an aqueous amine or physical solvent. The amine solution or solvent separates the residual H2S from the desorbed hydrocarbons. The newly rich solution containing polar gases can be recycled to natural gas or refinery stream clean-up processing. In this integrated process it has been found that the EXS molecular sieves and high silica aluminosilicate zeolites are useful adsorbents for removing the hydrocarbons from the Claus feed.